The Periodic Table PDF

The Periodic Table (1) Greek philosophers believed that the universe was made up of four main elements – Earth, Air, Wind and, Fire. There are two scientists who made a great contribution to the modern development of our understanding of the idea of an element. Robert Boyle - An Irish chemist who...

The Periodic Table (1) Greek philosophers believed that the universe was made up of four main elements – Earth, Air, Wind and, Fire. There are two scientists who made a great contribution to the modern development of our understanding of the idea of an element. Robert Boyle - An Irish chemist who gave the first accurate definition of an element in his book ‘The Sceptical Chemist’ (1600’s). He described an element as a simple substance that cannot be broken into anything simpler. **Definition: An element is a substance that cannot be split into simpler substances by chemical means.** Humphry Davy - An English chemist who discovered potassium by passing electricity through a compound of potassium (1807). - He also discovered the elements sodium, calcium, barium, strontium and, magnesium by passing electricity through compounds containing these elements. 1 History of the Periodic Table By 1800, approximately 35 elements had been discovered. Scientists began to look for patterns so they could memories the elements in a more structured way. Dobereiner’s Triads - In 1829, Johann Dobereiner, a German chemist, noticed that the properties of bromine seemed halfway between those of chlorine and iodine. - He noticed that the atomic weight of bromine (80) was about halfway between that of chlorine (37.5) and iodine (127). - He observed a similar pattern between calcium, strontium and, barium. He called these patterns triads. **Definition: A triad is a group of three elements with similar chemical properties in which the atomic weight of the middle element is approximately equal to the average of the other two.** Newland’s Octaves - In 1864, John Newland’s, an English chemist, arranged all known elements in order of their atomic weights. - He noticed that the properties of the elements seemed to repeat with every eight element. He stated this as his Law of Octaves. **Definition: Newland’s octaves are arrangements of elements in which the first and the eight element, counting from a particular element, have similar properties.** Mendeleev - In 1869, Dmitri Mendeleev, a Russian professor, wrote the properties of all known elements on separate cards. - He arranged the cards in order of increasing Atomic weight and placed elements with similar properties under each other in groups. - He noticed a periodic reoccurrence of properties. He called his findings his ‘Periodic Law’. **Definition: Mendeleev’s Periodic law – When elements are arranged in order of increasing atomic weight (relative atomic mass), the properties of the elements recur periodically.** - He placed all elements with similar properties in the same vertical column. - He left gaps in the table for elements that had not yet been discovered. - Mendeleev predicted the properties of the undiscovered elements. - He reversed the order of some elements to make them fall into groups of elements with similar properties. 2 Mosely – Atomic Number In 1913, Henry Mosely, an English chemist, studied the frequencies of the x-rays emitted by atoms of the different elements. - He found that frequencies varied with the quantity of positive charge on the nucleus. This was a method of determining the number of protons in the nucleus of an atom (atomic number). **Definition: The atomic number is the number of protons in the nucleus of that atom.** - Mosely showed that if elements are arranged in order of increasing atomic number it is not necessary to reverse the order of some elements - By arranging the elements in order of increasing atomic number the elements fall naturally into the correct groups. **Definitions: The modern periodic table is an arrangement of elements in order of increasing atomic number. The modern periodic law – When elements are arranged in order of increasing atomic number, the properties of the elements recur periodically.** The Modern Periodic Table - Elements are arranged in order of increasing atomic number. - 118 elements - Transition elements have their own block. - No gaps. Atomic Numbers and Mass Numbers **Definition: The mass number of an element is the sum of the number of protons and neutrons in the nucleus of the atom of that element.** - Sodium has 11 protons and 12 neutrons in its nucleus. Therefore, its mass number is 23 i.e., the mass of the atom is 23 atomic mass units. - Letter ‘Z’ is used to denote the atomic number while the letter ‘A’ is used to denote the mass number. Nuclear formula for Sodium. 23 is the Mass number (A) and 11 is the Atomic Number (Z). Number of Neutrons can be calculated with the following formula; Mass number (A) – Atomic Number (Z) = No. of Neutrons 23 – 11 = 12 3  Relative Atomic Mass, Isotopes and The Mass Spectrometer In 1919, Francis William Aston, an English chemist, built an instrument called a mass spectrometer to measure the masses of atoms. - He found that a sample of neon gas consisted of two varieties of neon atoms. One type of neon atom had a mass number of 20, the other type had a mass of 22. The atoms differed in the number of atoms in the nucleus. He called these isotopes of neon. **Definition: Isotopes are atoms of the same element, with the same atomic number, which have different mass numbers due to the different number of neutrons in the nucleus.** - Aston also determined the percentages of each of the isotopes present in a sample of the naturally occurring element. For example: A sample of Chlorine is found to consist of 75% 35Cl and 25% 37Cl. Calculate the average mass of an atom of chlorine. In 100 atoms of Chlorine there are 75 atoms of mass 35 = 75 x 35 = 2625 25 atoms of mass 37 = 25 x 37 = 925 Total mass of 100 atoms = 3550 Average mass of 1 atom = 35.5 **Definition: The average of the mass numbers of the isotopes of the element, as they occur naturally, taking their abundances into account and, expressed on a scale in which the atoms of the carbon-12 isotope have a mass of exactly 12 units.** Mass Spectrometer (2) 4 A mass spectrometer is used to separate and identify isotopes, measure their relative abundances, identify unknown compounds, and measure the relative atomic mass of an element. It can be used to detect banned drugs taken by athletes. It operates on the flowing basis: **Definition: The principle of mass spectrometry is that charged particles moving in a magnetic field are deflected to different extents according to their masses and are thus separated according to these masses.** - Vaporisation: A small amount of gas or liquid is injected into the sample inlet. Liquids turn to gases due to the vacuum inside the tube (they vaporise). - Ionisation: Atoms and molecules are turned into ions using an electron gun in an ionisation chamber. An electron gun is a heated filament which fires high energy electrons at the sample, knocking electrons off the sample and producing positive ions. - Acceleration: Positive ions pass through negatively charged plates accelerating the ions to high speeds. A fine beam of ions passes into the analyser unit. - Separating into a magnetic field: Positive ions are made to move in curved paths using a magnetic field produced by an electromagnet. The ions are separated according to their masses (Lighter particles are easier to deflect than heavier particles). - Detection: The positive ions fall on detector which responds to the number of ions hitting it. The signal is amplified into an electric current and viewed on a computer. The signal is sent to a printer where a series of peaks is printed out. This is called a mass spectrum. Writing Electron Configurations of Atoms Showing the electrons in the main energy levels of Potassium. Potassium is number 19 on the periodic table meaning it has 19 Protons and therefore, 19 Electrons. Remember, electrons must always occupy the lowest energy possible. K = 2, 8, 8, 1 Energy level 1 (n=1) can only hold two electrons Energy level 2 (n = 2) can only hold 8 electrons Energy level 3 (n=3) can only hold 8 electrons Energy Sublevels Energy level 1 (n=1) – no sublevels Energy level 2 (n=2) – two sublevels Energy level 3 (n=3) – three sublevels Energy level 4 (n=4) – four sublevels 5 The maximum number of electrons that can be occupied by an orbital is 2. Sublevel No. of Orbitals No. of electrons in sublevel s 1 2 p 3 6 d 5 10 **Definition: Aufbau Principle – When building up the electron configuration of an atom in its ground state, the electrons occupy the lowest available energy levels.** Writing the electronic configuration (s,p) for an atom of Calcium. Calcium is element number 20 so contains 20 electrons. d-Block Elements Since the 4s sublevel is lower in energy than the 3d sublevel, it must always be filled before the 3d sublevel. For example, Iron = 26 electrons Fe = 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6 - The d sublevel can hold 10 electrons. Therefore, the n=3 main energy level can hold 18 electrons. 2 electrons in the 3s sublevel, 6 electrons in the 3p sublevel, and 10 electrons in the 3d sublevel. Exceptions to the rule Chromium (Cr) has 24 electrons and is written as: Cr = 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5 This is because sublevels that are exactly half filled or completely filled are more stable. Therefore, one of the electrons in the 4s orbital flips over to one of the 3d orbitals to give two orbitals that are exactly half filled. Copper (Cu) has 29 electrons. Cu = 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d10 4s half filled and 3d completely filled to give extra stability. Electronic configurations on Ions An atom that has gained or lost electrons is called an ion. An atoms that gains electrons become negatively charged. An atom that loses electrons becomes positively charged. Write the electronic configuration of Al3+ ion. What neutral atom has the same electronic configuration. Aluminium is element number 13 so has 13 electrons. We can see from Al3+ that 6 the ion has lost three electrons, therefore, it contains 10 electrons. Al3+ = 1s2, 2s2, 3p6. This has the same number of electrons as neon. Write the electronic configuration of S2-. What neutral atom has the same electronic configuration. Sulphur is element number 16 so has 16 electrons. We can see from S2- that the ion has gained 2 electrons meaning that it now has 18 electrons. S2- = 1s2, 2s2, 2p6, 3s2, 3p6. This has the same number of electrons as Argon. Arrangement of Electrons in Orbitals of Equal Energy **Definition: Hund’s rule of Maximum Multiplicity states that when two or more orbitals of equal energy are available, the electrons occupy them singly before filling them in pairs.** For example, Nitrogen has 3 electrons in the 2p sublevel. px py pz One electron enters the px sublevel, one electron enters the pz sublevel and one electron enters the py sublevel. **Definition: The Pauli Exclusion Principle states that no more than two electrons may occupy an orbital and they must have opposite spin.** For example, Fluorine has 5 electrons in the 2p sublevel. 1s 2s 2p The upward facing arrow represents an electron spinning clockwise while the downward facing arrow represents an electron spinning anti-clockwise. Exam Questions 2019 – HL – Section B – Question 5 (i) What is meant by the periodic system in the context of Mendeleev’s 1869 periodic table of the elements? When elements are arranged in order of increasing atomic weight, the properties of the elements recur periodically. (ii) Comment on the positioning of tellurium (Te) and iodine (I) in the 1869 table. Iodine has a lower relative atomic mass than tellurium. So, iodine should be placed before tellurium in Mendeleev's tables. However, iodine has similar chemical properties to chlorine and bromine. To make iodine line up with chlorine and bromine in his table, Mendeleev swapped the positions of iodine and tellurium. (iii) Why did the 1869 table not include any noble gases? 7 They had not been discovered yet (b) Modern periodic tables arrange the elements in order of atomic number and not in order of relative atomic mass. (i) Define the underlined terms. - Atomic number is the number of protons in the nucleus of an atoms - Relative atomic mass is the average mass numbers of the isotope of the element as they occur naturally, taking their abundances into account and expressed on a scaled in which the atoms of carbon-12 isotope have a mass of exactly 12 units. (ii) Give an advantage of arranging the elements in order of atomic number. It means there is no need to reverse the order to force elements into their correct groups 10. (i) What is meant by an atomic energy level? The fixed amount of energy of an electron in an atom. (ii) Give two differences between an atomic orbit, as described by Bohr, and an atomic orbital. Orbit – Inconsistent with the wave properties of an electron and the uncertainty principle into account. Orbital – Electron can have wave properties and the uncertainty principle is taken into account. (iii) Use diagrams to distinguish between a p orbital and a p sub‐level (p subshell). p orbital p sub-level (iv) Write the electron configuration for a nickel atom in its ground state. 1s2 2s2 2px2 2py2 2pz2 3s2 3px2 3py2 3pz2 4s2 3d8 (v) State one piece of evidence that supports the existence of atomic energy levels Emission Spectrum 2013 – Hl – Section B – Question 4 Answer eight of the following items (a), (b), (c), etc. (a) Write the electron configuration (s, p, etc.) of a zinc atom in its ground state. 1s2 2s2 2p6 3s2 3p6 3d10 4s2 (b) Define relative atomic mass. The average mass numbers of the isotope of the element as they occur naturally, taking their abundances into account and expressed on a scaled in which the atoms of carbon-12 isotope have a mass of exactly 12 units. 8 References 1. Encyclopaediabrittanicia.co.uk 2. Chemguide.co.uk 9